I've had it happen twice in the past 7 months. The first time, my wife and I were coming back from St. Thomas. I was on a paid fare and she was on a FF ticket. The check-in counter lady asked if we wanted first class so we of course said yes. That was on a 757 and first was filled. The second time we were coming back from Cancun. We were both on FF tickets and the check-in lady again asked if we wanted first class. This was on a 321 and there were still plenty of first class seats left when we departed. The flight attendents even let 4 people come up from Y after the door closed. Neither time did we request the upgrade.

We've also been on FF tix to/from St. Maartin but were essentially laughed at for asking about upgrades. (This was on our honeymoon and the only time we've tried asking.) This was during the summer and the flights were completely full. So, I think it's all about luck but agree that coming back from the islands/Mexico, your chances are better.